We investigated biochemically and immunohistochemically the age-related changes in collagen (types I, III and IV), laminin and tenascin of the extracellular matrix in the submandibular gland of infant rats. The hydroxyproline content markedly increased on day 14 after birth. Electrophoretic profile revealed alpha chains of only type I collagen up to day six after birth. Type III collagen, which was diffusely distributed in interlobular connective tissue, was gradually expressed from day seven after birth. Type IV collagen content increased from day seven. Laminin content gradually increased with age. Tenascin immunohistochemically showed weak expression only around the vessel walls up to day seven. In addition, this expression markedly increased around acinar and ductal walls as basement membrane components on day 14 after birth. These findings suggest that rat submandibular gland types I and IV collagen, and laminin are already present during the fetal stage and relate to construction of tissue, and that type III collagen and tenascin, which increased from day seven after birth, relate to maturation.